The present invention relates to a device for compressing video data, and more particularly, to a device for compressing still image data to obtain a still image of high image quality.
Generally, a digital camcorder functionally provides a still image photographing mode as well as a moving picture photographing mode. In the moving picture photographing mode, video data is compressed frame by frame and continuously recorded in a recording medium, while in the still image photographing mode, a frame of video data is stored in a frame buffer, compressed, and recorded on a tape for several to tens of seconds.
The human eye is more sensitive to the quality of a reproduced still image than that of a reproduced moving picture, since the reproduced image is displayed in a still state.
Also, in many cases, a photographed still image is output according to its uses by equipment (e.g., a color printer) capable of displaying an image of high resolution, thus increasing the need for high image quality.
FIG. 1 is a schematic block diagram of a conventional device for compressing still image data.
The still-image data compression device of FIG. 1 is comprised of a discrete cosine transformer 10 for discrete-cosine transforming and zigzag-scanning video data received in 8.times.8 block units from a frame buffer (not shown) in a digital camcorder, a quantizer 12 for quantizing the video data output from discrete cosine transformer 10 at predetermined quantization intervals, a quantization interval determiner 16 for determining a quantizer number according to the complexity of the video data output in block units from discrete cosine transformer 10, so that quantizer 12 selects an adequate quantization interval or step size for each of macroblocks per second, and a variable-length coder 14 for variable-length-coding the video data quantized at the predetermined quantization intervals in quantizer 12, and outputting the variable-length-coded data to an error correction coder (not shown).
The quantizer number output from quantization interval determiner 16 has a direct impact on the determination of the image quality. In the prior art of FIG. 1, however, an identical quantizer number is selected to determine a quantization interval in both the still image photographing mode and the moving picture photographing mode.
As a rule, errors are typically present in video data processed in variable-length coder 14. Nevertheless, despite the presence of an error in the value of a quantizer number selected for the data of the first frame, a high-quality image can be reproduced only if a more accurate quantizer number is selected for the next frame data on the basis of the comparison of a reference bit number with the bit number of the first frame data, since several frames per second possess identical video data in a still image.
In the prior art, however, high image quality cannot be obtained since an identical quantizer number selected in a moving picture photographing mode is also used in a still image photographing mode.